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A USB Grounding Adapter
My Mac Mini has no provisions for grounding
and that is a problem. This cable solved the problem.
Warning: This project should
only be attempted by persons familiar with AC wiring and
relevant construction techniques. An error in assembling
the USB Grounding Adapter can result in grave danger of
bodily injury, death, or damage to equipment. Please
take this warning seriously.
The USB grounding adapter is merely a very short USB
extension cable and a very
short three prong AC extension cord with a wire to connect
their ground connections to one-another.
My use of USB on my Mac Mini
is not typical of most. Beyond the typical computer
peripherals such as a printer, scanner, mouse and keyboard,
I often have it connected to a microcontroller programmer or
to breadboard being tested.
Several month ago my first Mac Mini stopped working for the
last time. It was seven years old and was starting to show
its age, so while the expense of replacement was not
welcome, moving up to a newer model that was supported by
the current version of the Mac OSX operating system and with
twice the memory was welcomed.
Upon opening the box I was frustrated to see that there was
no ground connection on the power cable. It seems that
somebody at Apple can't make up his mind as to whether
grounding is good or evil. I've seen the ground conductor
come, go, come and go again.
To be fair to Apple, the typical user of a Mac Mini would
not notice any difference between a grounded chassis and an
ungrounded one because the user would rarely come in contact
with the chassis -the outside of everything including the
computer itself is insulated with plastic. Even if a user
did come in contact with a conductive part of the computer,
very strict safety requirements limit the maximum current
that could pass through the user's body to an annoying sting
or "buzz" as I have heard it called.
That leakage current is not good for the circuits on my
workbench. The first sign of trouble was the destruction of
an in-circuit programmer I had been using without trouble
for 10 years. But with the ungrounded Mac, the programmer
was "fried" beyond repair. Eventually I obtained a
replacement for the programmer.
Not long after that, a microcontroller in a breadboard
became damage, apparently from the same grounding issue and
had to be discarded. Wary of this, I started to be more
careful this time about the sequence of plugging things
Then a couple of weeks ago, I destroyed two more
microcontrollers. Before switching to the unused Mac,
destroying microcontrollers were ver rare events, and now it
started to look like the problem was going to be very inconvenient,
and a little bit expensive. I had procrastinated
enough and did what I knew I would have to do as soon as I
saw the ungrounded power cord: I made an adapter to ground
the Mac Mini.
Pin 4 is ground, and the
wire color is supposed to
be black or blue. Best to check with an ohmmeter first.
There are few places on the Mac Mini that give access to
"ground" but the ground connection on the USB cable is one, and
it was when using USB that I had the problems, so that seemed to
be the logical place to make the ground connection, and also
since it was the point at which the Mini's ground would meet
other grounds, it seemed to be the place that would have the
lowest chance of causing damage to the Mini's circuitry. Since
the Mini only has four external USB ports, I used a short USB
extension cable with the ground wire tapped so that I would not
loose use of the port. The ground connection was brought out of
the USB extension cable on the heavier green green wire
seen in the photograph. The green ground wire is two meters of
0.5 square mm stranded copper wire.